Friction is a force that slows down or stops moving objects when two surfaces rub against each other. It always acts in the opposite direction of motion. Rough surfaces (like sandpaper) create more friction, while smooth surfaces (like tile) create less. Friction is useful for walking, running, holding objects, and stopping vehicles using brakes. It also keeps things in place, like books on a table. However, friction can cause wear and tear on shoes, tires, and machine parts, and make it harder to move heavy objects. Friction also creates heat, which can sometimes be helpful (warming hands) or harmful (overheating machines). Lubricants like oil are used to reduce friction in machines and make movement easier.